Preferred Label : enthalpy of activation;
Detailed label : enthalpy of activation, Δ‡H ;
IUPAC definition : The standard enthalpy of activation Δ‡H is the enthalpy change that appears in the
thermodynamic form of the rate equation obtained from conventional transition state
theory. This equation is only correct for a first order reaction, for which the rate
constant has the dimension reciprocal time. For a second order reaction, for which
the rate constant has the dimension (reciprocal time) (reciprocal concentration),
the left hand side should be read as kc , where c denotes the standard concentration
(usually mol dm-3). \[k \frac{k_{{B}}\ T}{h}\ {e} {\frac{Δ {\ddagger }S {\,\unicode{x26ac}}}{R}}\
\mathrm{e} {\frac{-Δ {\ddagger }H {\,\unicode{x26ac}}}{R\ T}}\] The quantity Δ‡S
is the standard entropy of activation, and care must be taken with standard states.
In this equation kB is the Boltzmann constant, T the absolute temperature, h the Planck
constant, and R the gas constant. The enthalpy of activation is approximately equal
to the activation energy; the conversion of one into the other depends on the molecularity.
The enthalpy of activation is always the standard quantity, although the word standard
and the superscript on the symbol are often omitted. The symbol is frequently (but
incorrectly) written ΔH‡, where the standard symbol is omitted and the ‡ is placed
after the H.;
Origin ID : E02142;
See also
The standard enthalpy of activation Δ‡H is the enthalpy change that appears in the
thermodynamic form of the rate equation obtained from conventional transition state
theory. This equation is only correct for a first order reaction, for which the rate
constant has the dimension reciprocal time. For a second order reaction, for which
the rate constant has the dimension (reciprocal time) (reciprocal concentration),
the left hand side should be read as kc , where c denotes the standard concentration
(usually mol dm-3). \[k \frac{k_{{B}}\ T}{h}\ {e} {\frac{Δ {\ddagger }S {\,\unicode{x26ac}}}{R}}\
\mathrm{e} {\frac{-Δ {\ddagger }H {\,\unicode{x26ac}}}{R\ T}}\] The quantity Δ‡S
is the standard entropy of activation, and care must be taken with standard states.
In this equation kB is the Boltzmann constant, T the absolute temperature, h the Planck
constant, and R the gas constant. The enthalpy of activation is approximately equal
to the activation energy; the conversion of one into the other depends on the molecularity.
The enthalpy of activation is always the standard quantity, although the word standard
and the superscript on the symbol are often omitted. The symbol is frequently (but
incorrectly) written ΔH‡, where the standard symbol is omitted and the ‡ is placed
after the H.
